1. Field of the Invention
The present invention relates to a valve for controlling fluid flow, and more particularly to a flow regulating valve adapted to be inserted into a conduit for conveying oil under pressure from a reservoir to an oil-filled electric cable.
2. Discussion of the Prior Art
Regulating valves are used in conjunction with oil-filled electric cables to provide an oil flow limited to a fixed calibrated level when the cable is functioning normally, and to provide a variable flow, in case of partial or total lesions damaging the cable itself.
In case of partial or total rupture of the cable it is necessary for the valve to regulate and transmit flows over an ample range of flow rates. At first, elevated flows at calibrated levels are required for protecting the cable during a transitional thermal phase of cooling and thereafter the flow rate should be gradually decreased down to a value not greater than what is strictly necessary for preventing the entry into the cable of foreign bodies present in the ambient. On the other hand, very precise regulation of flow at very low rates allows for limiting to the indispensable minimum the outflow of oil that, for the long periods elapsing between the damage and the repair, as in the case of submarine cables, can be the cause of great economic losses and possibly of considerable problems of pollution.
Among the valves that are known and generally employed, attention is particularly directed to the valve described in applicant's prior South African Pat. No. 77/2626 granted on July 5, 1978 which presents numerous advantages over other kinds of known valves, principally in the regulation of flows of fluid over an ample range of values, as well as flow regulation with great precision at very low flow rates.
The valve of South African Pat. No. 77/2626 comprises a valve body provided with a through cavity, entrance and exit apertures, and an opening with variable opening area for varying the rate of flow of liquid passing through the valve. Inside the through cavity there is a deformable and impermeable membrane that divides the cavity into two compartments sealed with respect to each other, the membrane having means for varying the size of the opening, a conduit being provided inside the valve body which acts as a fluid communication link between the two compartments and fluid flow control means in the conduit, which, when operated, operates means for varying the size of the opening.
Fixed to the base of the membrane and facing the exit conduit are disposed means capable of varying the area of the opening, which means are constituted generally by a conical-shaped plunger that can be inserted more or less deeply, but never totally, into the opening itself and spring means urging the plunger away from the opening. These means of varying the area of the opening are controlled by two forces that act in the axial direction: a first force determined by the pressure difference that is established between the two compartments through the action of the fluid flow control means; and a second force determined by the difference between the pressure values upstream and downstream of the valve. This second force is effectively resisted by the action of the spring means. The advantages of this prior art valve are derived from the employment of the deformable and impermeable membrane that divides the through cavity into two fluid-tight compartments which are in communication by means of a conduit within which the fluid flow control means act. This arrangement makes it possible to adopt extremely variable correlations between the control surface of the membrane subjected to the difference of pressure between the two compartments, and the fluid flow regulated by the flow control means, permitting controlled flow over a very ample range of flow rates and with great precision, even for very small flow rates.
Although the valve described is quite satisfactory and reliable for numerous and varied applications, it is not completely suitable for certain extreme operating conditions, as when the difference between the fluid pressures upstream and downstream of the valve becomes, for any reason whatsoever, very high, especially when the pressure downstream is less than one fifth of the pressure prevailing upstream of the valve.
Under such extreme conditions the axial force acting on the means capable of varying the area of the opening can become so strong as no longer to be effectively counteracted by the spring means. Hence, there is the danger that in these extreme conditions, the conical shaped end of the plunger may close the opening more than necessary, and may even actually block the fluid flow.
It is accordingly an object of this invention to provide a valve that, while presenting all the advantages of the valve described in the cited South African Pat. No. 77/2626 presents the further advantage of functioning with absolute precision and reliability, even under extreme operating conditions, i.e. when the fluid pressure downstream of the valve is very low compared to the pressure upstream.